Constipation

Constipation is a slowing or arrest of bowel motility resulting in infrequent, difficult, or incomplete defecation.

As a drug effect it is dose-proportional and sustained for the duration of exposure, which distinguishes it from a pre-existing functional condition. It ranges from a mild slowing of transit to near-complete bowel stasis, with prolonged exposure deepening both severity and recovery time.

It is an uncommon adverse effect overall, though it is a defining feature of opioid use. It is most commonly induced at moderate to high doses under opioids such as heroin, fentanyl, and codeine, and under deliriants such as datura and myristicin. It also appears under depressants like clonidine and methaqualone, and under antidepressants such as mirtazapine and tianeptine.

Constipation is felt as a heaviness and pressure in the lower abdomen: a persistent fullness that does not resolve with the usual cues to defecate. Straining yields little or nothing, and the urge, when it does arise, is dull and unsatisfying.

It commonly comes with a sense of distension, generalized abdominal discomfort, and reduced appetite. Attending to the bloat tends to heighten discomfort without resolving it. At higher doses under opioids, motility can slow to the point where days pass without a bowel movement.

Constipation arises through different pathways depending on the drug class:

• Opioids — bind to mu-opioid receptors in the enteric nervous system, suppressing peristalsis and raising smooth-muscle tone throughout the gut; this effect does not habituate as readily as analgesia does.
• Deliriants — block muscarinic acetylcholine receptors, reducing the parasympathetic drive that sustains coordinated gut motility.
• Depressants — suppress peripheral autonomic signaling broadly, slowing the rhythmic contractions that move contents through the bowel.
• Antidepressants — alter serotonin and histamine activity in the gut, shifting motility signaling toward slowing.

How much each pathway contributes in a given case is not fully understood.

Who experiences it, and how strongly, varies widely. Dose is the strongest predictor — constipation is uncommon at low doses and becomes near-universal at high doses under opioids. Chronicity compounds dose; short exposures often produce only mild slowing, while sustained use deepens the effect markedly.

People with already-slow gut transit, low dietary fiber, or low habitual fluid intake are more susceptible. Adequate hydration and fiber intake reduce severity; tolerance to the gut effect develops slowly if at all.